The mechanism of calcium transport across the intestinal epithelial cell and the factors which control these processes in health and disease will be the subject of these investigations. Specifically, efforts will be continued in the characterization of the sodium dependent, parathyroid hormone sensitive enzyme of the basal-lateral plasma membrane proposed to mediate the active calcium transport at this site. In addition, the rate of vitamin D in the transport processes will be studied with particular emphasis on the trophic action of the vitamin and the metabolite or metabolites which mediate the observed biological responses. The biological responses will be examined at both the tissue and molecular levels. Finally, the influence of glucocorticoids and uremia on the transport processes and their interaction with mechanism of vitamin D and parathyroid action on the intestine will be investigated.